Im trying to build for .035" quench. The problem is that the pistons have some rock to them. When pro builders say build for .035" quench is that based on the average of the piston rock or are they basing the .035 quench on the maxium piston rock location? My pistons on average are .004 out of the deck, but you can rock them to +.016 above to -.008 below deck. So for proper quench do I buy .039 head gaskets or .051?

did you try checking them right above the wrist pin the area that the piston would rock the least? imo i would run the .051 gaskets just to be on the safe side. will not make much of a power difference.

Agree with miller_time, it might be fine with .039" gasket at .035" quench but you are really riding the line there with no real benefit over the .047 you'd have with the .051" gasket.

Most guys measure over the pin axis, where rock is the least, and .040" is generally considered the low limit that allows for that rock you are measuring to not be a factor.

I've run .034" quench on a 7500 rpm 383 stroker, but piston to bore clearance was .0035", seems like rock was only about .012" total in that case, and the pistons come so close to the head that the carbon is literally "buffed" see through on the quench side of the piston, we couldn't go any tighter without a collision.

Im trying to build for .035" quench. The problem is that the pistons have some rock to them. When pro builders say build for .035" quench is that based on the average of the piston rock or are they basing the .035 quench on the maxium piston rock location? My pistons on average are .004 out of the deck, but you can rock them to +.016 above to -.008 below deck. So for proper quench do I buy .039 head gaskets or .051?

Are you building for the street or the track? Will the pistons be forged or cast, what will be the skirt clearance? What will be the measured crankshaft main and rod bearing to journal clearances. What is the rod material, steel, titanium, or aluminum?

These are some of the basic questions that need to be answered when the squish/quench clearance starts getting below .040 inch. Tighter squish/quench is within the pursuit of absolute maximum power, as such the collision risk between piston and head goes up. Other problems arise in sealing the head and block as the gaskets get quite thin even on a zero deck block. Often the only sealing solution becomes O rings to maintain reliability. Engines running this tight usually are not expected to run too many miles between disassembly inspections and part replacements, this is something not practical for street engines that are expected to run several thousand miles between rebuilds.

While this is the territory of measuring piston rock and its effect on the squish/quench clearance, it also gets into the machining dimensions of the crank journals, rod lengths, and piston pin to crown, and crank centerline to head deck of the block. This requires that every cylinder be measured as to height to the deck as getting under .040 inch for squish/quench is well inside the range of production machining variation. This can force you into blue printing the individual parts which gets expensive very quickly. So in the end it comes to what the engine will be used for and how large is your Visa account maximum.

I understand all that but it doesnt really answer my question. Building for the street or strip or for a grocery go getter isnt going to make any difference if quench is based on the maximum piston rock and I build it for average.

If I take the average and build for .035 then that makes the max piston rock at only .024"..... Are quench numbers usually based on using the average of piston rock (center of piston front and back locations not the top and bottom) or based on using the maximum piston rock? Do they take into account piston rock when calculating for min quench numbers?

As far as i know most shops (and myself) measure over the pin, with .040" clearance being the accepted minumum that allows for acceptable clearance when running, and covers including the rock you are seeing. Remember that rock will be a lot less when the piston is up to operating temp and expanded to its actual running size.

I understand all that but it doesnt really answer my question. Building for the street or strip or for a grocery go getter isnt going to make any difference if quench is based on the maximum piston rock and I build it for average.

If I take the average and build for .035 then that makes the max piston rock at only .024"..... Are quench numbers usually based on using the average of piston rock (center of piston front and back locations not the top and bottom) or based on using the maximum piston rock? Do they take into account piston rock when calculating for min quench numbers?

It is the amount of squish/quench and clearance between the piston and head that is the question. A race engine is usually configured around the minimum possible clearance with 2 assumptions about race engines those being winning requires taking the engine to the absolute limits. Two, a race engine sees a lot of disassembly and internal maintenance so to a certain extent potential clearance issues that develop as the engine wears will be found before they become destructive. For street engine that's expected to wear for several 10's of thousands of miles without internal maintenance, one does not push the clearances to their limit. In this case the engine is not blueprinted so significant dimensional variance can exist between cylinders, so setting up one cylinder at the minimum may not be adequate at another cylinder.

The problem of squish/quench amounts is dependent upon area and clearance. Clearance gets a lot of talk, but area is seldom mentioned. There are two purposes to squish/quench. Squish is the ejection of mixture from the far side of the cylinder toward the spark plug. This serves to stir the mixture getting air and fuel molecules into close association and to break up any globules of fuel. This stirs and shoots the mixture into the space by the spark plug increasing the density of mixture before the plug. This increases the likelihood that the spark will catch the mixture on fire and once lit speeds the burn and takes the burn to a complete reaction which maximizes power and minimizes fuel consumption.

The other function is quench, this is the result of having a considerable amount of surface area enclosing a small volume. This removes heat from the far-side mixture so it does not self ignite before the flame front gets there. If it does self ignite there are at lease two pressure waves in the cylinder which when they collide make the typical pinging sound associated with detonation and/or preignition. This is the sound of the piston being hammered to death.

So far as I have seen there is no magic formula to determine the precise amount of squish/quench an engine should have. I've not seen a formula or rule of thumb in either Ricardo's or Taylor's writings.

By experience and documented tests we can see that the amount of squish/quench in terms of a function is directly proportional to cylinder diameter and spark plug placement. That is the larger the cylinder the more area of the combustion chamber needs to be devoted to Squish/Quench. This also holds for the further from the center the spark plug is located the more squish/quench is needed.

All this can be is an optimization of conditions. In the end there is so much resistance to detonation a given fuel has. The best you can do is get the most energy from the fuel without blasting the guts out of the engine. But you can't just get better and better performance from the fuel by making the squish/quench step bigger or closing the clearance. Ideally a modern combustion chamber is about 1/3 squish/quench and 2/3's valve and spark plug pocket for a wedge chamber. The spark plug located as close as practical to the center but needs to be off set enough for valve clearance. A hemi or pent roof chamber offers better spark plug placement but usually has a less effective squish/quench which is now a narrow band around the circumference of the cylinder.

Piston shape is very important, for a wedge a tight chamber with a flat top piston is ideal but usually creates too much compression. A D dish design allows the compression ratio to be tailored and keeps the dish under the valve pocket. The OEMs have a bad habit of using circular dish pistons because of cost advantages they fit both sides of the engine where D dishes have a separate left and right orientation and therefore from a cost stand point only half as many of a design orientation can be made. The circualr dish reduces the close locating of the piston crown to the squish/quench deck reducing the functional area. The bottom of the dish may sit from .08 to .1 inch lower than the crown, seriously limiting the amount of closure clearance. These things significantly reduce this function in most production engines which pushes the owner operator to compensate with a higher grade fuel or more of it as in rich mixtures, or compromising the ignition timing thus throwing unburnt mixture out the tail pipe power and efficiency going with it.

Looking at the piston rock issue from a non technical side, if the piston is rocked to either side, it will be rocked back as it reaches TDC, in effect 'leveling' itself somewhat, the area over the pin axis is where any <solid> contact will occur if it's going to happen, therefore where quench should be set. JMO.

? when the piston comes up on the compression stroke, why wouldn't the pressure be equal over the top of the piston? Somebody's law says pressure inside a confined space would be equal in all directions, so why would the piston be level?

? when the piston comes up on the compression stroke, why wouldn't the pressure be equal over the top of the piston? Somebody's law says pressure inside a confined space would be equal in all directions, so why would the piston be level?

Thanks, that's what I was trying to say. Did you mean to say why 'wouldn't' the piston be level?

As far as i know most shops (and myself) measure over the pin, with .040" clearance being the accepted minumum that allows for acceptable clearance when running, and covers including the rock you are seeing. Remember that rock will be a lot less when the piston is up to operating temp and expanded to its actual running size.

Thanks, that's what I was trying to say. Did you mean to say why 'wouldn't' the piston be level?

No it is not level going over the top. First a shape, other than flat, offers changes in area exposed to the pressure. Then different crown shapes change the weight from one side of the pin center to the other. Then the flame front applies force in a progressive way across the bore twisting the piston on the pin, at ignition there is more force being applied by the spark plug than on the far side of the cylinder. This tends to lift the piston on squish quench side. Then, nearly all pistons offset the pin from the piston center causing them to ride higher on one side which varies as to the side of the engine. This is done to reduce the snap over the piston makes as it changes direction at TDC and BDC it is intended to take some shock load off the skirt. So the pistons usually are operating in a slightly kocked over position all the time.

Add to those movements the dynamic of the piston weight going over the top. This wants to and does stretch the rod length however minor that may be, but it also pulls the rod bearing clearance up adding a few more thousandths inch to everything.

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